Re: Proper use of templated containers as class members

Maxim Yegorushkin <>
Thu, 4 Dec 2008 08:37:53 -0800 (PST)
On Dec 4, 4:29 pm, Jeff Schwab <> wrote:

Maxim Yegorushkin wrote:

On Dec 4, 3:15 pm, Victor Bazarov <> wrote:

Per wrote:

I am finding myself doing the following rather often when I have
template containers in classed, e.g. stl::map
For example:
class Foo
   typedef std::map<std::string, int> direcory_t;
   //More typedefs of the same style...
   directory_t directory_m;
   //More members of the same style...
   //Constructors and stuff
   const directory_t& directory() const {return directory_m;}
The question is what is your opinion on typedefing like this. At the
same time as it saves a lot of typing. After a while there are very
many types in a project. And in the end can get confusing about what
the type really contains. Classes should of course hide their
implementation but it seams very stupid not to have read access
directly to a member. If I didn't have that I have to bloat the class
with wrapper functions for the members. And not using typedefs would
lead to an awful lot of typing.

I use this approach everywhere. It's an abstraction. The users o=

f your

class will use "Foo::directory_t" when they need the type. The main
thing is not saving typing (although it's a nice side effect) but the
ability to change what 'directory_t' means with a single line, and not
to worry about changing the rest of the code that uses it. That's t=


whole idea behind typedefs. It doesn't matter how they are defined,=


a namespace member or as a class member.

I also use this technique quite a bit, though usually with iterators,
rather than container types.

More often such an approach is called abstraction leak.

There is the potential for leakage under certain circumstances, but
there is no abstraction leak in the posted code. For you to list any
such problem with the code, you're going to have to start with "what if..=


The interface of directory_t "abstraction" is that of std::map<>. You
can only change the type of directory_t to something that supports the
full interface of std::map<>, otherwise you break the client code (you
may be lucky if there was limited use of std::map<> directory_t
interface, so that recompilation after changing the type of
directory_t succeeds, but you can't count on that).

The problem you're describing applies when the amount of client code
that can access the exposed type is unbounded. When the code is alread=


encapsulated within some intermediate layer, e.g. it is an
implementation detail of some stand-alone application, there is no need
to enforce any tighter enapsulation. The public typedef for the
otherwise private type is worth having in such a scenario, not because
it limits potential uses of the type, but because it serves to identify

The literal text "std::map<std::string,int> tells the reader what type
is in use, but Foo::directory_t gives the more relevant information of
why this is the right type for the job. If ever you do need to change
the type's interface, it's a lot easier to search for Foo::directory_t
than to figure out which instances of std::map<std::string,int> are
relevant to Foo.

If you break the interface in a way that the compiler catches, within
the context of implementing an application, it's not a particularly big
deal. The potential to break the interface in ways the compiler cannot
catch, e.g. changing the big-O complexity of an operation, is no less
prevalent if a custom type is used rather than a typedef.

My point was that it was not an abstraction, because it abstracts away
nothing but the name of the type. Rather a convenience typedef to make
code less fragile.

Sorry for nor being clear on this.


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